How to enable Steam family sharing
Steam Families is a feature in Valve’s Steam game distribution client that allows you to share your personal game library (the games you have purchased from the Steam service) with friends and family.
The system is not without limitations, however. Only one device or user can access the library at one time. The account owner is always given preference, but that means if you share your account with your child, only one of you can use the library at a time.